Poly(trimethylene terephthalate) is a polyester which can be prepared by esterification of 1,3-propanediol and terephthalic acid or dimethyl terephthalate, and polycondensation thereof.
Processes for producing poly(trimethylene terephthalate) have been published several times. For example, U.S. Pat. No. 6,277,947 discloses a process for producing poly(trimethylene terephthalate) by esterification of terephthalic acid with trimethylene glycol in the presence of a catalytic titanium compound, pre-condensation, and polycondensation. Here, initially, the esterification is carried out by at least two steps under the conditions of a mole ratio of trimethylene glycol to terephthalic acid of 1.25 to 2.5, a titanium content of 0 to 40 ppm, a temperature of 245 to 260° C., and a pressure of 1 to 3.5 bar, and 35 to 110 ppm more titanium than in the initial step is added thereto in the at least one subsequent step.
Further, U.S. Pat. No. 6,353,062 discloses a continuous 3-vessel, 3-stage process for preparing poly(trimethylene terephthalate), wherein the first vessel is an esterification reactor for preparing a mixture of bis(3-hydroxypropyl)terephthalate and low molecular weight oligomers, the second vessel is a prepolymerizer, and the third reactor is a final polymerizer or finisher.
Furthermore, U.S. Pat. Nos. 7,132,484 and 6,538,076 disclose a continuous 4-vessel, 4-stage process for preparing poly(trimethylene terephthalate), wherein the first vessel is an esterification reactor for preparing a mixture of bis(3-hydroxypropyl)terephthalate and low molecular weight oligomers, the second vessel is a flasher, the third vessel is a prepolymerizer, and the fourth reactor is a final polymerizer.
In addition, U.S. Pat. No. 5,599,900 discloses a method for eliminating by-products by bringing dihydroxy trimethylene terephthalate or a low molecular weight oligomer thereof which are obtained by esterification of terephthalic acid or dimethyl terephthalate and 1,3-propanediol in contact with an inert gas in the process of a polymerization reaction.
Meanwhile, by-products such as acrolein and allyl alcohol are formed in the preparation process of poly(trimethylene terephthalate). However, these by-products are highly toxic and thus it is preferable to minimize the formation of toxic by-products in the preparation process of poly(trimethylene terephthalate).
There have been several methods known for reducing toxic by-products such as acrolein in the process of preparing poly(trimethylene terephthalate). For example, in order to reduce the production of toxic by-products, U.S. Pat. No. 7,381,787 discloses a method of adding a phosphorus compound; U.S. Pat. No. 6,093,786 discloses a method of adding a hindered phenol and an aromatic organophosphite together; and U.S. Pat. No. 7,223,471 discloses a method of adding polyfunctional alcohols, anhydrides of carboxylic acids, carboxylic acids and their salts, carbohydrates, and so on to the poly(trimethylene terephthalate) melt.
However, the methods of reducing by-products using additional additives as in such known methods not only complicate the production process but may also form other unexpected by-products, and particularly, there is a problem of deteriorating the physical properties of the final polymer.